Optical discs such as CD, MD, and DVD comprise an information recording area formed from a pattern of fine depressions and protrusions (pits and/or grooves), which are formed in accordance with a predetermined format defined for each disc.
In order to measure the degree of eccentricity relative to the center of rotation for this optical disc pattern of fine depressions and protrusions, conventionally a mechanical precision measuring device such as an eccentricity measuring device or an axial displacement measuring device has been used.
A variety of mechanical precision measuring devices have been proposed.
It is a known representative structure in which a laser beam is irradiated through an objective lens that can be driven by an actuator, on the fine depressions and protrusions of the optical disc. In the structure, the laser beam tracks the depressions and protrusions, so that the movement (the degree of displacement) of the objective lens, which reflects the displacement in a radial direction (the eccentricity) of the fine depressions and protrusions, is detected as a fluctuation in the beam spot position on an optical position sensor (for example, see Japanese Patent Laid-Open Publication No. 1987-117151).
In recent years, optical discs capable of recording and playback using optical systems that use blue lasers have been proposed, including the DVR system which uses an optical system with laser light of wavelength 405 nm, and a NA value of 0.85.
However, conventional mechanical precision measuring devices used for CD or DVD discs have been produced on the basis of CD or DVD format properties (for example, a laser light wavelength λ of 780 nm, a numerical aperture NA=0.45, and a groove pitch of 1.6 μm in the case of CD, and 650 nm, NA=0.6, and a groove pitch of 0.74 nm in the case of DVD). Therefore, if the groove pitch of the pattern of fine depressions and protrusions is reduced to 500 nm or less for example, then the pattern becomes difficult to track so that these conventional devices cannot be adopted as measuring devices for DVR.
Of course, the eccentricity of these types of high density optical discs can be measured by using a DVR (higher performance) mechanical precision measuring device having similar structure to that for CD or DVD.
However, these types of mechanical precision measuring devices are generally expensive, even for CD media. Products for DVR media are even more expensive. If a new device is required for each new format, then large costs are unavoidable. Moreover, another problem arises in that if a new measuring device is purchased, the existing mechanical precision measuring devices used for measuring CD or DVD serve as a surplus.
On the other hand, the measurement and evaluation of optical disc eccentricity by inspection under a microscope, not by measurement with a mechanical precision measuring device, has also been frequently performed. However, even in these microscope measurement methods, when the groove pitch of the pattern of fine depressions and protrusions is reduced to 500 nm or less, the required light diffraction phenomenon is hard to be obtained so that the measurement itself becomes more difficult.